The present invention relates to a high-reliability AC coupling receiver circuit for a serial data communication system using balanced transmission lines, and to a transmitter circuit including a receiver circuit of this kind.
To improve the reliability of a data communication system, it is known to connect various units forming a communication system to one another by a balanced transmission line, e.g., a twisted pair having a pair of electric wires twisted around each other, deliver signals of opposite phases to the paired electric wires, respectively, and detect the differential between the signals as a transmission signal. This type of communication system is used also for a control system of a motor vehicle. To be used with a motor vehicle wherein the protection of human life must be given top priority, a data communication system is not only required to be satisfactorily durable and low in manufacturing cost, but also required to have high reliability, i.e., fail-safe functions, under various environments. For example, even in the event of an abnormal condition that the voltage of one of the paired transmission lines is fixed at a constant voltage, e.g., an earth potential, a power supply voltage, etc. or is opened due to the disconnection of a connector or the like, or that a short circuit occurs between the transmission lines, normal transmission functions of the various units of the system must be retained.
A bus driver/receiver of a distributed multiplex transmission system for motor vehicles is conventionally known which is equipped with a receiver circuit of the type determining the presence/absence of an input signal to the transmission lines (i.e., passive or dominant state of the transmission lines) by the voltage difference between the balanced transmission lines (see SAE, RECOMMENDED PRACTICE J--1850). In this arrangement, however, if the voltage of one transmission line is fixed at an earth potential, for example, the output logic level of the paired transmission lines is also fixed even though the logic level of the signals applied to the other transmission line varies normally, thus failing to achieve normal receiving functions and to satisfy the above-mentioned requirements for an automobile system. In view of this, an AC coupling receiver circuit is conventionally used in which a capacitor is arranged before a receiver circuit connected to the transmission lines, to enable normal reception of signals based on the output logic of a normal transmission line whose voltage is not fixed, e.g., at the earth potential, even in the case of the above-mentioned trouble. By simply connecting the capacitor, however, the direct-current level of the transmission signal changes as the signal passes through the capacitor, depending on the duty ratio of the signal, whereby the received threshold voltage to be applied to a signal processing circuit next to the capacitor varies and thus normal signal reception cannot be performed. Therefore, in this AC coupling system, measures must be taken to eliminate the fluctuation of the direct-current level, in particular, the fluctuation of the received threshold voltage caused by the charge stored in the capacitor during serial pulse transmission. For example, an arrangement illustrated in FIG. 1 is conventionally used in which signal voltages at the two signal lines are applied via an AC coupling to a differential amplifier, which preferably comprises a high-speed operational amplifier, and the voltage differential between the two signals, delivered from the differential amplifier, is subjected to level shift by another AC coupling and then is compared with a reference voltage.
According to this prior art arrangement, however, a differential amplifier comprising, e.g., a high-speed operational amplifier, must be arranged after the AC coupling, which leads to an increase in the number of component parts of the circuit and accordingly in cost and also requires a large space for arrangement.